Hanging Outbound TDM Channels

I am experiencing hanging outbound channels in Asterisk 1.4.22 with Aculab TDM cards (but as to the experience I have made the last days I presume that can happen with Zap as wel).

I use a Local channel to proxy outbound calls over an outbound context to diversify routes. Like Dial(Local/123456@routeout) to dial outbound and use my “routeout” context to use different trunk groups per tech prefix (e.g. 1x => Dial(Aculab/g1/${EXTEN}) and 2x => Dial(Aculab/g2/${EXTEN}

In the following case below the outbound Aculab channel AND the Dial(Local…) channel both hang:
channel transition IDLE -> WAIT_FOR_OUTBOUND -> PROGRESS -> and then IDLE again (called pty unpolitely rejects my call, for some reason)

In “core show channels” the Aculab channel shows state “Down” in “AppDial((Outgoing Line))”, and the Local channel hangs in state “Ring” in e.g. Dial(Aculab/r2/4311503)

From the debug file I can see that the Aculab channel driver has been asked to hangup the call. The associated inbound leg has been hung up also.

I tried now for days to debug Asterisk channel code and the Aculab channel driver, but with no success. Any ideas? Any experiences? Your help is very much appreciated!